Foamed polymeric materials are well known, and typically are produced by introducing a physical blowing agent into a molten polymeric stream, mixing the blowing agent with the polymer, and extruding the mixture into the atmosphere while shaping the mixture. Exposure to atmospheric conditions causes the blowing agent to gasify, thereby forming cells in the polymer. Under some conditions the cells can be made to remain isolated, and a closed-cell foamed material results. Under other, typically more violent foaming conditions, the cells rupture or become interconnected and an open-cell material results. As an alternative to a physical blowing agent, a chemical blowing agent can be used which undergoes chemical decomposition in the polymer material causing formation of a gas.
U.S. Patent No. 3,796,779 (Greenberg; Mar. 12, 1976) describes injection of a gas into a flowing stream of molten plastic, and expansion to produce a foam. The described technique typically produces voids or cells within the plastic that are relatively large, for example on the order of 100 microns or greater. The number of voids or cells per unit volume of material typically is relatively low according to that technique and often the material exhibits a non- uniform distribution of cells throughout the material. Therefore, thin sheets and sheets having very smooth finishes typically cannot be made by the technique, and materials produced typically have relatively low mechanical strengths and toughness.
U.S. Pat. No. 4,548,775 (Hayashi, et al.) describes a technique involving extruding an expandable resin through a plurality of holes bored in a die and then fusing together material extruded from the holes. The technique is designed to form a high-density skin layer on the foamed material since, according to Hayashi, et al., with single-hole dies the extrudate is deformed by foaming after the material leaves the die and it is not possible to form a skin layer uniformly.
U.S. Pat. No. 3,624,192 (McCoy, et al.) disclose extrusion of thermoplastic polyaromatic resin, admixed with a nucleating agent, through a network of slits to form a foam board.
U.S. Pat. No. 3,720,572 (Soda, et al.) disclose production of “synthetic wood” defined by an elongated microporous article formed of a plurality of coalesced, foamed resin strands. Borders between the strands mimic wood grain, which is the object of the invention.
U.S. Pat. No. 4,473,665 (Martini-Vvedensky, et al.; Sep. 25, 1984) describes a process for making foamed polymer having cells less than about 100 microns in diameter. In the technique of Martini-Vvedensky, et al., a material precursor is saturated with a blowing agent, the material is placed under high pressure, and the pressure is rapidly dropped to nucleate the blowing agent and to allow the formation of cells. The material then is frozen rapidly to maintain a desired distribution of microcells.
U.S. Pat. No. 5,158,986 (Cha, et al.; Oct. 27, 1992) describes formation of microcellular polymeric material using a supercritical fluid as a blowing agent. In a batch process of Cha, et al., a plastic article is submerged at pressure in supercritical fluid for a period of time, and then quickly returned to ambient conditions creating a solubility change and nucleation. In a continuous process, a polymeric sheet is extruded, then run through rollers in a container of supercritical fluid at high pressure, and then exposed quickly to ambient conditions. In another continuous process, a supercritical fluid-saturated molten polymeric stream is established. The stream is rapidly heated, and the resulting thermodynamic instability (solubility change) creates sites of nucleation, while the system is maintained under pressure preventing significant growth of cells. The material then is injected into a mold cavity where pressure is reduced and cells are allowed to grow.
In continuous extrusion processes in general, typical goals involve high production rates (flow rates), production of material having a desired shape, size, material density and cell density, especially materials having relatively thin or thick portions, and production of materials with a highly smooth surface. In all cases, of course, it is a goal to produce material at the lowest possible cost. While conventional foam processing can operate at very high output rates, typical known continuous microcellular extrusion production rates do not approach the rates achievable with conventional processes. In conventional foam polymer processing, a desired shape, size and density of a product generally can be achieved using a conventional shaping die. However, extruding very thin material or very thick microcellular material can be difficult. With respect to thick sheets, it has been difficult or impossible to create the necessary solubility change uniformly throughout a thick product produced by extrusion to produce a thick microcellular article continuously. With respect to thin sheets, where the cell size is large relative to the thickness of the sheet, small holes in the sheet can develop where a particular cell is of a dimension larger than the thickness of a sheet. Additional control problems exist in many known thin foam sheet extrusion techniques. Accordingly, it has been a challenge to extrude thin coatings of conventional foam cellular material onto substrates such as wire. In particular, where a substrate such as wire must be isolated from moisture, if a foam material is to be used to coat the substrate then the foam should be essentially completely closed-cell material. Therefore, it has been difficult or impossible to extrude thin, closed-cell polymeric material onto wire to form a coating having acceptable electrical insulation properties under various conditions.
Traditionally, chlorofluorocarbons (CFC's), hydrochlorofluorocarbons, (HCFC's), and alkanes (butane, pentane, isopentane) have been used as blowing agents to produce foam products. These agents reportedly provided superior foaming control, as they reportedly are partially soluble in polymers, acting as plasticizers to lower the glass transition temperature (Tg) of the material, thereby reducing melt viscosity and permitting process cooling of the extruder melt as necessary to obtain foam physical characteristics such as mechanical strength, smooth foam, and unruptured cells. In part due to environmental problems associated with these agents, however, effort has been directed towards the use of low environmental impact atmospheric gases such as carbon dioxide, nitrogen, and air as blowing agents, and success has been met in some cases (see, e.g., U.S. Pat. No. 5,158,986 (Cha), above). But successful control during foaming with atmospheric gases has been more difficult to achieve than with conventional agents. Some references report that the solubility of atmospheric gases in polymers is inherently lower than conventional blowing agents, therefore Tg and melt viscosity are not reduced to the same degree, necessitating relatively higher processing temperatures when using atmospheric gases in order to maintain necessary melt flow. Higher processing and melt temperatures can produce reduced polymer melt strength as compared to similar conditions using conventional blowing agents, resulting, in many cases, in explosive cell expansion upon release of the melt to atmosphere.
In some instances, control in atmospheric gas blowing agent processes has been addressed with high temperature melt processing during the incorporation of the blowing agent, followed by melt cooling prior to extrusion and foaming to increase melt strength. In particular, several patents and publications focusing on foaming of amorphous polymers using solely carbon dioxide as blowing agent have stressed criticality of melt and/or die temperature do not exceed a particular temperature.
For example, U.S. Pat. No.4,436,679 issued to Winstead on Mar. 13, 1984, and U.S. Pat. Nos. 5,266,605 and 5,250,577 issued to Welsh on November 30, 1993 and Oct. 5, 1993, respectively, disclose cooling prior to the extrusion of amorphous polymer foams formed using solely carbon dioxide blowing agent. European Patent Application EP 0 707 935 A2 published Apr. 24,1996 by Baumgart et al. (Assignee BASF) describes extrusion of amorphous polymeric material with a large temperature drop to control extrusion.
Due to the process and material limitations described above, and in particular temperature limitations, those of ordinary skill in the art would not expect to achieve highly-controlled, high volume microcellular processing of crystalline and semi-crystalline polymers, especially when using atmospheric gases. Crystalline and semi-crystalline polymers differ from amorphous materials in that they have a distinct crystalline melting temperature (Tm) that is much higher than their glass transition temperature. If cooled to Tm, these materials will abruptly solidify, making further processing impossible. Prior to this abrupt solidification, the melt strength of the polymer will not increase appreciably with increased cooling, as in the case of amorphous polymers, because the temperature of the polymer is necessarily so much higher than Tg. That is, crystalline and semi-crystalline polymers must be processed at temperatures well above (relative to Tg) ceiling temperature for amorphous polymers, driving cell expansion and making it extremely difficult to maintain small cell sizes.
Therefore, the production of microcellular material using atmospheric gases has focused primarily on amorphous polymers, which become viscous and flow easily at temperatures above Tg.
While the above and other reports represent several techniques associated with the manufacture of microcellular material, there is a need in the industry for a viable continuous method of producing crystalline and semi-crystalline microcellular material.
It is, therefore, an object of the invention to provide a high-throughput, continuous, microcellular or supermicrocellular polymer extrusion system effective in producing microcellular material of high quality and in any of a variety of desired thicknesses, in producing microcellular material as a coating for wire, and in producing high-quality crystalline and semi- crystalline microcellular material.